Looking Up: Seeing double - Gemini’s stars

Peter Becker

Friday

Feb 25, 2011 at 12:01 AMFeb 25, 2011 at 4:40 PM

Astronomy column: Riding high overhead on late winter evenings is the constellation Gemini, the "twins." This pattern of stars is marked by two bright stars, Castor and Pollux, which aren’t exactly identical twins.

Riding high overhead on late winter evenings is the constellation Gemini, the "twins." This pattern of stars is marked by two bright stars, Castor and Pollux, which aren’t exactly identical twins.

To find this pair of luminaries, face south and look upward and to the left of the bright constellation Orion, marked by the famous trio of “Orion’s Belt.” At the upper left corner of Orion is the brilliant, fiery red-orange star Betelgeuse. Just off to the left is the beginning of the constellation Gemini. Castor and Pollux are off to the far left.

You can also start with the brightest star in the night sky, Sirius, which shines to the lower left of Orion. Move up and a little left to the bright yellow star Procyon, and then up again, to Pollux and Castor above that.

Pollux is of magnitude +1.15 and appears yellow-orange. The relatively cool star is single, having no star accompanying it through the galaxy. Lest you feel sorry for lone Pollux, take heart. Like the sun, which is also “single,” Pollux is known to have at least one planet in orbit (the sun has a bunch). At least 2.3 times the mass of Jupiter, the planet moves around Pollux once in about 589 days. Pollux lies about 33.7 light years from the sun (about 195 TRILLION miles).

Castor, by contrast, is a much hotter star, shining blue-white, and is the brightest component of a multiple star system. The star isn’t quite as bright to our eyes, shining at magnitude +1.96 and is about 15 lights years farther than Pollux (49.8 light years).

A relatively small telescope will reveal that Castor is resolved into two. The individual stars shine as magnitude +2.8 and +2.0. They orbit about a common point once every 467 years. Each of these stars have been found to be close double stars, making Castor not twins, but quadruplets. In addition, this quartet of stars is accompanied by another close pair of stars, making the entire system of six stars, sextuplets!

The constellation is along the ecliptic, the imaginary line around the entire sky, where the sun appears to travel. The solar system is mostly close to the same flat plane, like dots marked on a compact disc with the sun in center. As a result, all the planets from Mercury to Neptune, and the moon, never venture far from the ecliptic. Thus, the planets take their turns passing through Gemini, as does the moon.

The Milky Way Band passes through this part of the sky. On a clear, moonless night, try scanning this area with binoculars or a small telescope. You will find a great spread of faint stars, making up the Milky Way. A large, bright star cluster is found in Gemini, known as M35. Appearing as a faint patch in binoculars, partly resolved into stars, the cluster really opens up in a small telescope. Use a low power eyepiece, to keep the whole extent of the cluster in the field of view.

In mid-December, a very strong meteor shower known as the Geminids, appear to radiate from this constellation.

New moon is on March 4, giving us dark, moonless nights this week.

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